The present invention relates in general to probes for accessing points on small integrated circuit (IC) elements fabricated on semiconductive or dielectric wafers, and in particular to a system for providing an electrical connection between probes accessing the same IC element.
Wafer probes provide temporary electrical contacts between test equipment and the very small bonding pads of planar devices such as IC elements fabricated on semiconductive wafers. Use of wafer probes permits operation and testing of IC elements prior to separating, bonding and packaging the individual IC elements on a wafer. However, many existing wafer probes do not permit accurate measurement of high frequency (e.g. 2 GHz and above) signals appearing on an IC element because the signal conduction paths provided by the probes to connect bonding pads on the IC elements to measurement equipment do not have constant impedance. Impedance mismatches in a signal path cause signal reflections which lead to errors in signal measurements.
Copending U.S. application Ser. No. 605,462, filed Apr. 30, 1984, describes a wafer probe comprising coplanar ground and signal conductors deposited on the under side of a tapered alumina substrate, each signal conductor and its adjacent ground conductor forming a separate, constant impedance transmission line. The ground conductors are all electrically interconnected by a metallic channel which spans the signal conductors without touching them. Each transmission line extends from a coaxial cable connector at the wide end of the substrate to contact pads at the narrow end of the substrate, the contact pads being arranged to align with bonding pads on an IC element. This probe is particularly useful for accessing IC elements in which the bonding pads conveying signals to be accessed appear in a row and the row includes at least one additional bonding pad connected to ground. Since each transmission line of the probe includes a signal conductor and a ground conductor, and since the ground conductors are all interconnected by the channel, the constant impedance environment of each transmission line extends from the cable connector to the IC element even when only one ground conductor of one transmission line contacts a grounded bonding pad on the IC element.
IC elements often contain two parallel rows of bonding pads, and in such case each row may be accessed by a separate probe. However in many IC elements, a grounded bonding pad appears in only one of the rows of bonding pads and not in the other row, and therefore the ground conductors of one of the two probes cannot directly access ground on the IC element. One might solder a jumper wire between tips of ground conductors of the two probes so as to permit ground conductors in one probe to indirectly access ground through the other probe, thereby permitting the constant impedance transmission lines provided by the first probe to extend to the IC element without interruption. However, the probes are small and a jumper wire capable of interconnecting ground conductors of the two probes would be very small, difficult to install, and easy to break.